[Mp4-tech] H.264 frame rate

[Gary Sullivan]

Further clarification: 
Those constraints that I mentioned that are specified in Annex A apply only to the following:
1) conformance of timing relationships expressed in (or with) bitstreams, and
2) decoders that satisfy output timing conformance requirements.
They do not apply to decoders that only satisfy output order conformance requirements.
It may also be worth remarking that (Annex C): "In order to check conformance of a bitstream using the HRD [specified in Annex C], all sequence parameter sets and picture parameters sets referred to in the VCL NAL units, and corresponding buffering period and picture timing SEI messages shall be conveyed to the HRD, in a timely manner, either in the bitstream (by non-VCL NAL units), or by other means not specified in this Recommendation | International Standard."
The allowance of "other means not specified" allows timing information to be conveyed or otherwise specified by system-level or application-level methods instead of by syntax within the bitstream.  However, it may mean that the "naked" bitstream used in such a system cannot be tested for conformance without providing this important information somehow.  Thus, providing this information somehow is important in system/application design work.  Without that information the bitstream is incomplete (just as a bitstream that does not contain SPS and PPS NAL units is incomplete, although such bitstreams can be used in some applications where such information is conveyed by external means not specified in the video coding standard).
Best Regards,
Gary Sullivan
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	From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Gary Sullivan
	Sent: Tuesday, August 14, 2023 10:58 AM
	To: ChenJianjun; mp4-tech lists.mpegif.org
	Subject: RE: [Mp4-tech] H.264 frame rate
	Please define the term frame rate.  Are you under the impression that the number of decoded pictures per second needs to be a constant?  If so, that is an incorrect impression in some use cases.
	If there is no frame rate expressed in the video elementary stream syntax, there may be such an animal defined in a system-level or application-level specification.
	If there isn't, then it is possible that the video bitstream does not have a frame rate (depending on your definition of what a frame rate is).
	However, there will be a constraint on how close together in time the decoder can be required to decode each pair of consecutive pictures in the bitstream in decoding order.  Similarly there will be a constraint on how close together in time the decoder can be required to output each pair of consecutive pictures in the bitstream in output order.  Those constraints are specified by the level constraints in Annex A.
	Best Regards,
	Gary Sullivan
________________________________
		From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of ChenJianjun
		Sent: Tuesday, August 14, 2023 5:41 AM
		To: mp4-tech lists.mpegif.org
		Subject: [Mp4-tech] H.264 frame rate
		Dear experts:
		How to calculate the H.264 frame rate? 
		I noted that VUI parameter(SPEC E.1) and Picture timing SEI message semantics(SPEC D.2.2) may have such information.But they are optional,then how to get the frame rate information if there's no such information?
		Thanks!
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